The interleukins comprise a family of polypeptides derived from cells of haematopoietic origins that regulate not only the growth and differentiation of haematopoiesis but also the clonal expansion and maturation of the immune system. Although many of these rgulatory proteins have been molecularly closed, relatively little is known about their intracellular mechanism(s) of action in regulating S phase progression or gene expression. We have examined several parameters of cellular signal transduction initiated by ligand-receptor interactions of interleukin 2 (IL2) and interleukin 3 (IL 3). Among the early intracellular events initiated by each ligand on their respective cloned interleukin-dependent cell lines are i) the rapid mobilization of calcium, ii) stimulation of phospahtidylinositol turnover, iii) subcellular redistribution of protein kinase C and, iv) the rapid phosphorylation of a number of proteins which are substrates of protein kinase C. Since protein phosphorylation has been long regarded as a requisite mechanism of regulating protein /enzyme functions, the identity of the substrates as well as the phosphotransferase network involved becomes critical for understanding the biochemical basis of growth regulation and gene expression. IL 2 and IL 3 both stimulated the phosphorylation of a 68 kd protein within the cytosol of their target cells, suggesting that both ligands may share identifcal transmembrance signalling apparatus involving the 68 kd protein kinase C substrate. Among the membrane associated proteins prosporylated by PK-C is the IL 2 receptor. PK-C activation is also associated by IL-2 stimulation of mRNA synthesis of gamma interferon and Tac epitope mRNA.